Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.
Waste heat can be converted into useful energy by a variety of turbine generator or heat engine systems that employ thermodynamic methods, such as Rankine cycles. Rankine cycles and similar thermodynamic methods are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine, turbo, or other expander connected to an electric generator or pump. An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa). More recently, in view of issues such as thermal instability, toxicity, flammability, and production cost of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate non-hydrocarbon working fluids, such as ammonia.
During a typical startup procedure, various components of the heat engine system begin to warm up, and the flow of the working fluid through a working fluid circuit is initiated. However, the waste heat flue is usually immediately operational at the beginning of the startup procedure. The thermal energy in the waste heat stream may cause immediate heat soaking of a heat exchanger provided to transfer heat from the waste heat stream to the working fluid. If the working fluid absorbs excess energy from the heat exchanger during the startup procedure, the properties of the working fluid may be disadvantageously altered, and one or more components of the heat engine system may be subject to damage or wear.
For example, if the working fluid absorbs excess thermal energy, then the working fluid may change to a different state of matter that is outside the scope of the system design. For further example, if a generator system requires the working fluid in a supercritical state, once overheated, the working fluid may have a subcritical, gaseous, or other state. Further, the overheated working fluid may escape by rupturing seals, valves, conduits, and connectors throughout the generally closed generator system, thus causing damage and expense. Additionally, the increased thermal stress can cause failure of fragile mechanical parts of the turbine power generator system. For example, the fins or blades of a turbo or turbine unit in the generator system may crack and disintegrate upon exposure to too much heat and stress. An overspeed situation is another expected problem upon the absorption of too much thermal energy by the turbine power generator system. During an overspeed situation, the rotational speed of the power turbine, the power generator, and/or the drive shaft becomes too fast and further accelerates the flow and increases the temperature of the working fluid and, if not controlled, generally leads to catastrophic system failure.
Additional concerns may arise during the startup procedure because the working fluid may change from a vapor phase to a liquid phase on a low pressure side of the fluid circuit, and the pressure of the liquid must be raised on the high pressure side of the circuit. Raising the pressure of a liquid phase by pumping generally requires less work per unit mass of working fluid than raising the pressure of a vapor phase by compression, and pumping also results in a higher overall cycle efficiency. Unfortunately, one consequence of pumping is that bubbles may form if the working fluid drops below the saturation temperature and pressure for the specific working fluid. Such bubbles may cause or otherwise form cavitation of the pump used to circulate the working fluid in the fluid circuit, thus leading to flow reduction and, in some cases, catastrophic damage to the pump and shutdown of the heat engine system.
Therefore, there is a need for systems and methods for generating electrical energy in which temperatures and pressures within a working fluid circuit are controlled to reduce or eliminate thermal stress on vulnerable mechanical parts of the heat engine system during a startup procedure.